Interventional Radiological (IR) procedures are gaining importance as cost effective methods to effect therapeutic actions within the body. In these procedures a physician attempts to guide a catheter along the vascular network to a remote internal site within the patient using real-time fluoroscopic imagery for visual feedback. A fluoroscope system consists of an X-ray source positioned below a patient lying on a table and a two-dimensional detection screen coupled to a two-dimensional image intensifier positioned above the patient. A standard fluoroscope produces an equispaced temporal sequence of radiographic images of whatever lies between the X-ray source and the detection screen. A manually adjusted set of shutters at the X-ray source (positioned at the start of the procedure) determines the direction and spatial extent of the beam. The X-ray beam intensity is adjustable either manually or in some commercial systems, semi-automatically, to give proper image exposure. The patient/physician dosage is determined jointly by the beam intensity, the area of the beam, and the number of frames taken.
The trend is to attempt increasingly complicated procedures that require traversing the vascular system to increasingly remote sites within the body. The fluoroscope is the primary tool that the physician uses to enable him to thread a catheter to a remote internal site. It allows him to visualize the internal vasculature and to gauge the position and orientation of the catheter relative to the vasculature. As the destination site becomes more remote, the time to get there increases rapidly. The fluoroscope typically is energized for a sizable percentage of the total procedure time, resulting in significant accumulated patient X-ray dosage. For the patient this will probably be an isolated dose. However, the physician, because he performs many procedures per year, can accumulate a dangerous dosage level. The physicians can adjust the system operating parameters to limit the dosage to the absolute minimum while maintaining sufficient image quality to allow manipulation, but in many cases existing systems do not provide entirely risk-free use.
Existing systems expose a large area of the patient to the X-ray beam to provide the physician with a full field of view on each exposure. Typical systems acquire images every 33 milliseconds resulting in an extremely large accumulated dosage of X-rays for patient and doctor.
Some existing systems adjust the beam current of the X-ray tube on a frame by frame basis based on measuring overall image brightness of the most current image. Typically, the mean value of the brightness histogram is used to raise or lower beam strength. The analysis is entirely global and does not depend on recognizing and tracking local scene activity or content. Thus, what is required is a fluoroscope system that automatically and continuously adjusts the system parameters (X-ray beam shape, application point, beam intensity, and the time interval between exposures) by real-time analysis of the images produced by the system in such a way as to preserve maximum fidelity of manipulation information while reducing the X-ray dosage to a minimum.